Known in the art is a spark ignition type internal combustion engine provided with a variable compression ratio mechanism able to change a mechanical compression ratio and a variable valve timing mechanism able to control a closing timing of an intake valve, performing a supercharging action by a supercharger at the time of engine medium load operation and engine high load operation, and increasing the mechanical compression ratio and delaying the closing timing of the intake valve as the engine load becomes lower in the state holding the actual combustion ratio constant when the engine operation is shifted from the high load operation to the medium load operation (for example, see Japanese Patent Publication (A) No. 2004-218522).
However, this document does not allude at all to the actual compression ratio when the engine load is low.